small diameter coil

[wirelessguy]

I have an application for a specialized metal detector coil. I want to use a Whites MXT to provide ferrous/non-ferrous discrimination information on targets which are deeper in quartz than common metal detector depths (2-30 feet). These targets were not found with found a metal detector.

I want to insert a metal detector probe encased in PVC pipe into a 2” dia hole bored into the quartz.

Reading Carl’s “Coil Basics” tutorial at http://www.geotech1.com/cgi-bin/page...&file=info.dat it seems to me one of the figure 8 configurations would work well in this application. I would insert the probe on adequate length sections of PVC into the drilled hole and connect to the MXT.

I’m armed with no metal detector probe building experience, a lot of RF experience, adequate test equipment.

Will this idea work in general?
Is one of the figure 8 configs the right topology?
Any other hints on making this probe before I start?
What distance from centerline of 2” dia drilled hole can I expect to discriminate on a US nickel sized object for ferrous/non-ferrous?

[satdaveuk]

Perfect project for your requirements

Hi Wireless guy
This set-up would fit the bill for your job, Have constructed 7 to date, 5 of which have been sold to customers, so proved repeatable construction keeping to tight specifications hence performance.
This design is PI format, non motion, with hi/bright blue visual led that mimics adjustable threshold output as like the audio.
Working voltage 9V-12V DC with built in reverse polarity protection.
one set of 8 AA or battery Pk full charge will give 2 days life on typical use.
The actual coil shown is 1.5" O/D which is, installed into the coil housing then sealed.
The stem is standard small bore waste pipe, stem length up to 2.5meters/ 9FT long, the coil cable feeds through the stem so as not to get damaged and plugs into the belt hip mounted detector housing via standard type whites 5 pin plug/socket.
Metal presents indication is via standard HI-FI headphones or LED or both.
Power is via either Rechargeable Battery pack or dry cells which are also hip mounted and connected via a lead terminated into DC plug/socket
My tests were done using 1.5" coil as described on typical pasture land here in the UK which showed good indication at 6.5" with a buried 22ct solid gold ring. Second test was with a 1929 solid silver 6d coin which came in at 6" loud and clear.
Its PI design cloned from original Barracuda design , constructed with precision value components.
The main project was posted here in the early days but since added enhancements such as delay fine tune etc.
I normally use this particular design for finding and tracing underground electrical cables although with the 8" coil connected it makes a good deep seeking relic detector.
Pictures shown are pro types one of which was built into a housing built from cut up and glued scrap plastic, second and battery housing was bought from Farnell

If you want any more information prompt me with a email to sign in here Geotech because these days work is so hectic meaning I have not got the time to come on here as much as the old days
Regards
Dave